The present invention relates generally to micro-machined thermal accelerometers, and more specifically to an improved technique of compensating for sensitivity variations over temperature in thermal accelerometers.
Thermal accelerometers are known that have the capability of detecting acceleration along multiple axes. For example, U.S. Pat. No. 6,182,509 (the '509 patent) discloses a thermal accelerometer device configured to detect acceleration along 2-axes. As disclosed in the '509 patent, the 2-axes thermal accelerometer comprises a substrate having a cavity etched therein, and a structure including a small heater plate and four temperature sensors suspended over the cavity. The heater plate is positioned at the center of the suspended structure, which is in a plane defined by the X and Y axes. Further, two of the four temperature sensors are placed along the X axis on opposite sides of and at substantially equal distances from the heater plate, while the other two temperature sensors are similarly placed along the Y axis on opposite sides of and at substantially equal distances from the heater plate. In a typical mode of operation, electrical current is passed through the heater plate, which heats the surrounding fluid (e.g., air) within the cavity to generate a symmetrical temperature gradient in the directions of the X and Y axes. Because the respective pairs of temperature sensors disposed along the X and Y axes are equidistant from the heater plate, the differential temperature between each pair of temperature sensors is initially zero. However, if an accelerating force is applied to the device in a direction parallel to the X-Y plane, then the temperature distribution of the fluid shifts. For example, when acceleration is applied in the X direction, a differential temperature can be detected by the temperature sensors disposed along the X axis. Similarly, when acceleration is applied in the Y direction, a differential temperature can be detected by the temperature sensors disposed along the Y axis. A bridge circuit and a differential amplifier are typically employed to generate signals representing the detected differential temperatures, which are proportional to the acceleration applied in the directions of the respective axes. According to the '509 patent, the thermal accelerometer can be fabricated using known CMOS or bipolar processes, thereby providing a highly reliable accelerometer that can be integrated with signal conditioning circuitry at relatively low cost.
One drawback of the above-described thermal accelerometer is that its sensitivity generally depends upon the thermal properties of the fluid within the cavity of the device. Such thermal properties include the fluid density, specific heat, thermal conductivity, and dynamic viscosity. Each of these fluid thermal properties is a function of the temperature of the fluid, which depends upon the level of heat generated by the heater plate and the ambient temperature. Accordingly, to utilize the thermal accelerometer in applications in which the device is subject to significant fluctuations in ambient temperature, e.g., automotive applications, techniques must be employed to compensate for sensitivity variations over a range of temperature.
One technique of compensating for sensitivity variations over temperature in thermal accelerometers includes employing a micro-controller to access compensation values from a lookup table, and to correct the accelerometer output using the compensation values. Such a technique has drawbacks, however, because area limitations and implementation complexities can make integrating a thermal accelerometer with a micro-controller rather difficult. Compensation techniques that employ digital signal processing (DSP) are also problematic due to the difficulties involved in integrating DSP circuitry with a thermal accelerometer. In addition, compensation techniques employing external micro-controllers or DSP devices can be problematic due to accompanying increases in material and manufacturing costs.
Another drawback of conventional thermal accelerometers such as the thermal accelerometer described above is that they typically fail to provide a ratiometric compensation for variations in power supply voltage. As a result, an absolute reference voltage is generally required to implement read-out circuitry for these devices, resulting in increased implementation complexity and cost. In addition, conventional thermal accelerometers typically fail to provide self-test procedures, which are often required in applications demanding high levels of reliability, e.g., automotive and medical applications.
It would therefore be desirable to have a thermal accelerometer that provides a compensation for sensitivity variations over a range of temperature. Such a thermal accelerometer would also provide a ratiometric compensation for variations in power supply voltage and a self-test procedure, while avoiding the drawbacks of the above-described conventional thermal accelerometers.